Laser Doppler blood flow monitors have been used in conjunction with pneumatic cuffs to determine localized blood flow see for example The Correlation between Three Methods of Skin Perfusion Measurement (J. Vasc Surg 1992;15:823-30). Laser Doppler blood flow monitors are known from U.S. Pat. No. 4,596,254 among others. Devices of this type use a fiber optic wave guide to scatter coherent light off of both, moving particles, mainly red blood cells (RBC), and stationary vascular structures. The back scattered radiation is conducted by a separate fiber optic wave guide to a photodetector. The back scattered laser light returned to the photodetector is frequency shifted by the moving particles in accordance with the Doppler effect. Consequently the photodetector signal includes an AC signal component related to the number of RBC and their velocity. This AC signal is superimposed on a DC offset signal which depends in part on the intensity of the coherent light source and the geometry of the multiple fiber detector. Blood volume or hematocrit information and blood velocity information can be extracted from the composite photodetector signal.
Blood flow information is computed by multiplying the blood velocity data by the blood volume data. One problem with the use of flow monitors for perfusion measurements or studies is that the blood flow product value is very sensitive to small errors in measured blood velocity or measured blood volume. This property of flow measurement makes the technique problematical for monitoring skin perfusion pressure.